1. Field of the Invention
The present invention relates to a signal processing circuit. It more particularly relates to a radio frequency signal (hereinafter referred to as an "RF signal") processing circuit which is applicable to an optical recording and reproducing apparatus for an optical disk for information interchange.
2. Description of the Background Art
There are two different techniques which are employed for tracking control of an optical recording and reproducing system.
One is called a continuous servo method. In this system, signals are optically recorded in a continuous spiral groove or on the continuous land on a disk surface between the wraps of the spiral groove.
The other is called a sampled servo method In this system wobble marks (pits) for tracking control and one clock mark (pit) for synchronization are formed at the positions of servo bytes in a preliminarily formed track on the disk surface.
FIG. 5 illustrates a sample of the block diagram of a prior art RF signal processing circuit for an optical recording and reproducing apparatus with the continuous servo method. The reproduced signal from a pick-up head 9 is supplied to a head amplifier 1. The head 9 is radially movable adjacent to a rotating optical disk 10. The reproduced signal is boosted by the head amplifier 1 and then applied to the signal processing circuit 3 through a coupling capacitor 2. The signal processing circuit 3 operates upon the signal in order that it be easily decoded, for example equalizing it, and applies binary conversion to it against a predetermined reference level The output from the signal processing circuit 3 is inputted to a decoder 4. The address information included in the ID signal which was recorded beforehand in the form of a pre-pit, is derived from the output of the decoder 4.
There will now be described a process consisting of the steps of recording an information signal into a first block on the disk and reproducing the recorded signal from a second block adjacent the first block on the disk. The wave form of the RF signal applied to the head amplifier 1 through the pick-up head is illustrated in trace (a) of FIG. 6. At the pre-formatted ID area of the first block to be recorded, the ID signal is first reproduced. Then when the head is at the data area of the first block, other means than the pick-up part of the head are optically recording data in this data area. However, a signal which corresponds to the data being recorded is applied to the head amplifier 1 since the optical data signal being recorded is reflected from the disk surface and detected by the pick-up head. In the second block, the next ID and data signals are reproduced and applied to the head amplifier from the ID and data areas. Even though the signal level which is applied to the head amplifier 1 from the recorded signal by light reflection is high enough at the corresponding pit, the signal level which is applied from the reproduced signal in the corresponding pits of the second block is much lower. Due to this fact at the switching point of the process from signal recording to signal reproducing, the average signal level is stepwise changed. Though the situation is as described above, a DC component can be easily removed from the signal by selecting the coupling time constant of the capacitor 2, for example over 30 kHz as a high-pass cut-off frequency, because there is sufficient distance between the ID area and the data area of the preceding block. In this case, the RF signal output from the capacitor 2 is as shown in trace (b) of FIG. 6. Therefore, it is possible to detect and correctly reproduce the ID and data signals of the following block by the binary conversion of the RF signal with a corrected ground level and by the subsequent decoding.
FIG. 7 illustrates a sample of the block diagram of an RF signal processing circuit for an optical recording and reproducing apparatus using the sampled servo method of the prior art. An example of the RF signal wave form to be applied to the head amplifier 1 through the pick-up head is shown in trace (a) of FIG. 8. In this case, the blank signal from the unrecorded data area is detected and applied from the unrecorded first data block. A predetermined recorded data signal in the servo byte is detected by light reflection and applied to the head amplifier from the recorded second data block. Furthermore, an ID data signal is detected and applied from the third block. These signals are applied through a coupling capacitor 11 to an amplifier 12. Then the output from the amplifier 12 is introduced to an A/D converter 13 for conversion to digital form. In the case of the optical disk of the continuous servo tracking type, the data signal is converted by M2 modulation, which is different for the optical disk of the sampled servo tracking type, which relies upon, for example, 4-15 modulation. Therefore, in the signal processing for the sampled servo method, not only binary conversion but also analog to digital conversion is required.
In the optical disks using the sampled servo tracking method, the distance between the servo byte area and the data area is not great. In order to accurately maintain the original wave form, the cut-off frequency of the coupling capacitor 11 should be set at less than one-tenth of the sampling frequency, for example 41.3 kHz. In this context, the coupling time constant of the capacitor 11 should be large to a certain extent. Due to this fact when the signal is recorded with a large magnitude level optical beam, the average DC component of the output signal from the capacitor 11 rises for a while, as is illustrated in trace (b) in FIG. 8. In other words, the base line level of the RF signal from a reproduced data block, which is read after an adjacent data recorded block, rises from the ground level. Heretofore, to attain the effective A/D conversion even in such cases, the input dynamic range of the A/D converter 13 should be designed with a greater margin beyond the expected signal gain range extending from ground as the maximum level varies towards the negative side. This means that the resolution for the A/D conversion is degraded in the condition of a fixed dynamic range, and further means that the dynamic range must be expanded to obtain a predetermined level of resolution.